The present invention relates to a miniaturized accelerometer of the type using spring compensation of the gravity effect. It also relates to a process for the production of such an accelerometer.
The invention makes it possible to compensate the effects of gravity, to which the object is exposed, consequently improving its sensitivity to an acceleration variation. It more particularly applies to mechanical devices having small dimensions manufactured by microelectronic (e.g. micromachining) methods. The main field of application of said accelerometer is the study of the movement or behaviour of media subject to gravity (e.g. seismology).
An accelerometer or acceleration sensor is constituted by a seismic mass, which is generally supported by one or more flexible elements. When this mass undergoes acceleration variations, it is displaced and the flexible elements are deformed. The system returns to its initial position as soon as the force due to the acceleration is cancelled out. In the inoperative state, a horizontal acceleration sensor is subject to no force or stress. Conversely, a substantially vertically axed accelerometer is subject to a minimum, permanent force F, due to gravity, such that: EQU F=M.g
M representing the seismic mass and g the gravitational constant.
This permanent force is added to the signal to be measured and requires an increase in the dynamics of the sensor when it is wished to measure very small, vertical accelerations of below 10.sup.-6 g. It is consequently important in this case to compensate the force due to gravity by a constant force directed in the opposite direction.
The processes used for compensating the effect of gravity can be placed in two categories, namely those using an electric power source and those using the return force of a spring.
The processes using an electric power or energy source use an electro-static or electromagnetic force, which maintains the seismic mass in suspension. The systems with electrostatic or electromagnetic compensation of the force of gravity are complex and costly. They consume power and make use of servoloops, which are noise sources and whose stability is difficult to control, particularly in the case of electrostatic forces. Accelerometers produced using microelectronic methods hitherto only make use of electrostatic or electromagnetic compensating systems, despite the disadvantages thereof. This is explained by the fact that hitherto noone has proposed the simple solution of producing a stretched or drawn springs using collective methods.
The gravity force compensating processes using the return force of a spring are generally employed in equipment produced on the basis of conventional mechanical methods, i.e. by machining, followed by the individual assembly of the parts. However, FR-A-2 735 580 has recently proposed an accelerometer, whose seismic mass is maintained in equilibrium by a prestressed spring and which can be produced by methods based on mechanics, micromechanics or microelectronics. The compensating process proposed is based on the principle of a spring in blade or leaf form produced by prestressing a surface of an element (e.g. a beam) supporting the seismic mass.
Finally, mixed systems exist using both an electrostatic or electro-magnetic force and the return force of a spring, as disclosed in the article entitled "The Effects of Spring and Magnetic Distortions on Electromagnetic Geophones" by S J. Chen and K. CHEN, published in J. Phys. E.: Sci. Instrum., 21, 1988, pp 943-947.
The gravity force spring compensating process has the advantage, compared with the process using an electric power source, of not intro-ducing background noise produced by an automatic control system. Moreover, a compensation by spring constitutes a simple, stable, inexpensive and reliable process.
Acceleration sensors having a substantially vertical axis for which the effect of gravity on the mass is compensated by a spring are at present produced by assembling various mechanical parts. Due to this construction procedure, such devices do not have a very high quality factor Q. This structural parameter is linked with the Brown noise density S of the device by the following relation, which shows that S is inversely proportional to Q and M: ##EQU1##
In order to maintain a Brown noise which does not disturb the measurement, existing devices have a significant mass M. Nevertheless, this solution limits the miniaturization of the assembly. The smallest high performance means (able to detect a few nano G under 1 G) consequently weigh several kilograms and occupy a volume of a few dozen cm.sup.3.
The miniaturization of a high performance device makes it necessary to reduce the mass M and consequently increase the quality factor. This can be achieved by making the entire sensor (mass and spring) of a material having a high quality factor, such as e.g. monocrystalline silicon. However, the production of a compact device having a spring integral with the mass gives rise to technological problems. Thus, it is difficult to interconnect small mechanical parts, such as the spring and the seismic mass by mechanical means, such as screws or adhesive, without giving rise to areas where internal frictions are high and cause damping phenomena prejudicial to the quality factor. It is also necessary to maintain a considerable flexibility of the spring which, as is demonstrated by the following equations, influences the sensitivity of the sensor: ##EQU2## with s: flexibility of the spring
k: rigidity of the spring PA1 S: sensitivity PA1 K: rigidity of the device without compensating spring. PA1 a support, PA1 a seismic mass which can be exposed to a force induced by an acceleration to be measured, PA1 mechanical connection means between the support and the seismic mass, which can bend under the effect of said force, PA1 detection means making it possible to determine the acceleration on the basis of the force induced in the seismic mass, PA1 elastic means for compensating the force exerted on the seismic mass by gravity and linking the seismic mass and the support, characterized in that the support, the seismic mass, the mechanical connecting means and the elastic compensating means are elements produced in a same substrate, regulating means being provided for placing under mechanical tension the elastic compensating means, in order to compensate the force exerted on the seismic mass by gravity. PA1 a) etching the substrate in accordance with its thickness to define: PA1 b) placing the spring under mechanical tension by attaching the spring head to said other element, i.e. support or seismic mass, until there is a compensation of the force exerted on the seismic mass by gravity. PA1 a) etching the silicon layer until the silicon oxide layer is reached, in order to define: PA1 b) eliminating the silicon oxide layer below the elements other than the support, PA1 c) placing the spring under mechanical tension by attaching the spring head to said other element, i.e. support or seismic mass, until the force exerted on the seismic mass by gravity is compensated.